1. Field of the Invention
The invention relates to smectic liquid crystal devices.
2. Discussion of Prior Art
Liquid crystal devices commonly comprise a thin layer of a liquid crystal material contained between two glass plates or walls. A thin transparent electrode is deposited on the inner face of both walls. The combination of liquid crystal layer, walls, and electrode is often termed a liquid crystal cell. When an electric field is applied between the two electrodes, liquid crystal molecules rotate in the field to an ON state. On removal of the field the molecules rotate back to an OFF state, determined by a surface treatment applied to the walls before cell assembly and by the type of liquid crystal material. The optical transmission characteristics of the ON and OFF states are different. Some devices need one or two polarizers and/or dyes to visibly distinguish between ON and OFF states.
There are broadly three different types of liquid crystal material, nematic, cholesteric, and smectic, each distinguished by a different molecular ordering.
Such materials only show a liquid crystal phase over a limited temperature range between the solid and isotropic liquid phases. Within the liquid crystal phase temperature range a material may exhibit one or more of the nematic, cholesteric or smectic phase types. Normally a material is chosen such that it forms only one type of liquid crystal phase over its working temperature range.
The present invention concerns devices using smectic liquid crystal materials.
Displays have been made with the electrodes formed into rows on one wall and columns on the other wall. These collectively form an x, y matrix of separately addressable elements on a large display. One type of display uses the ON and OFF states to form an electrically switchable optical shutter. Yet another type of display is used as an optical storage device. Nematic cholesteric and smectic liquid crystal material have been used for such devices. A problem with many displays is the time taken to switch between the two states i.e. the response times. For many displays a fast response time is needed. A nematic material, arranged in a 90.degree. twisted structure typically has a response time of 100 milliseconds.
Devices comprising smectic materials have not been used as widely as devices with nematic or cholesteric materials. Available display devices based on smectic materials do not have the necessary characteristics. Recently however the smectic ferro electric property with its fast switching and bistability characteristics has become important see for example N. A. Clark & S. T. Lagerwall, App. Phys. Letters 36 (11) 1980 pp 899-901. Chiral liquid crystal materials in a tilted smectic phase, e.g. S.sub.C *, S.sub.I * , S.sub.F *, S.sub.H *, S.sub.J *, S.sub.G *, are known to exhibit ferroelectric properties. This has been described by R. B. Meyer, L Liebert, L. Strzelecki and P. Keller, J. de Physique (Lett), 36, L-69 (1975).
One problem with liquid crystal devices when used as a shutter is the amount of light that leaks through in its minimum transmission states. For example a twisted nematic cell may let through a minimum of about 1% of light. Reducing the minimum light transmission is highly desirable for shutter applications. Also it is useful in other displays for contrast enhancement. A further problem with twisted nematic displays and shutters is their narrow angle of view; when viewed away from the normal to a display the information displayed can be corrupted.
Ideally a liquid crystal shutter should have fast response times and a very low minimum light transmission. For a digital display the display should have high contrast and a wide angle of view.